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SSH on Mac OS X for Worry-Free Wireless

by Derrick Story and Rob Flickenger

11/21/2001

Network security for projects such as webcam broadcasting isn't as much of an issue when your device is hard-wired into the desktop computer sitting on your office desk. But if you set up a wireless transmitter using an 802.11b network as described in my last article, The Industrial-Strength Wireless Webcam, you need to evaluate your environment and establish the appropriate level of security.

Once you start sending private data over the radio waves, such as your FTP user name and password, you are increasing the likelihood that some unauthorized snoop can "listen in" and grab your data.

In most home 802.11b situations, I still believe that the combination of common sense and the use of WEP provides you with the security you need for broadcasting to your hobby Web site. But what if you want to use a webcam in public and send images to a business site? Certainly you don't want to take a chance on compromising security.

In situations like this, the Unix underpinnings of Mac OS X begin to show their value. In this article, I'm going to suggest a safe solution for broadcasting your webcam images, or any other data for that matter, using the Terminal application in Mac OS X to establish a secure connection with your Web server.

Since we're talking Unix here, these techniques will work in any command line environment capable of using SSH, and we're definitely not limited to using Mac OS X for this project. So let's pop the hood and get to work.

A few words about SSH and SCP

Comment on this articleDo those of you who have some experience using pico and Mac OS X have any additional pointers?
Post your comments

SSH Secure Shell is a protocol for secure remote logins. Its key function is to prevent hackers from stealing passwords that give them unauthorized access to Web sites. SSH does this by encrypting the data (including passwords) to eliminate eavesdropping. It connects to the specified hostname, making the user prove his/her identity to the remote machine.

Once the SSH connection is established, SCP (secure copy) copies files between hosts on a network. It uses SSH for data transfer, and uses the same authentication by asking for passwords or phrases.

Mac OS X users can set up these secure connections via the Terminal application. If you haven't worked with command lines before, I suggest that you you practice first by building a couple of basic scripts. You can get more information on this from a variety of places, such as the Project Freedom Web site, or via downloadable PDFs such as Griffman's Terminal Guide. Once you're comfortable with the Terminal application, then you're ready to set up a SSH session.

Basic steps for establishing a secure wireless session with a Mac OS X client

The process for establishing secure transmission of webcam images on a public wireless network is:

  • Create a directory on your local drive and tell CoolCam to send the captured images to that directory.
  • Generate an ssh public/private key pair.
  • Set up the ssh directory on the Web server, and copy the new public key to it.
  • Test a secure copy to ensure everything is working properly.
  • Set up a cron job so that your images are are automatically sent from the local directory to the server in regular intervals.
  • Disable the cron once your webcam session is over.

Setting up your secure session on Mac OS X

Related Articles

Network Test Automation with Mac OS X and Tcl


Industrial-Strength Wireless Webcam


Life After AirPort -- New Wireless Base Stations


Using SSH Tunneling


Once you have the webcam software saving to a file in the filesystem, you're ready to set up ssh for public key operation. Both ssh and scp use the same keys to do their work, so once ssh is set up, scpwill work.

These steps assume that you don't have any other keys present on your Web server. If you do, then you already know what you're doing, and don't need this article... =)

1) Generate a new key: 


rob@entropy$ ssh-keygen -d
Generating public/private dsa key pair.
Enter file in which to save the key (/home/rob/.ssh/id_dsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/rob/.ssh/id_dsa.
Your public key has been saved in /home/rob/.ssh/id_dsa.pub.
The key fingerprint is:
33:3c:5c:41:98:1b:fc:f5:9e:69:56:2e:0b:f1:24:7f rob@entropy

* The -d option specifies DSA keys (instead of RSA keys). The ssh v2 protocol uses DSA keys, and is widely regarded as more secure than v1.

* After entering the command, hit enter three times (to take the default filename, and to enter no passphrase.)

* Congratulations. Your public and private keys are now saved to ~/.ssh/id_dsa.pub and ~/.ssh/id_dsa, respectively.

2) Copy the key to your Web server:


rob@entropy$ scp ~/.ssh/id_dsa.pub www.mydomain.net:.ssh/authorized_keys2

* At this point, if you've never used ssh from your OS X box before, you'll be prompted to verify the fingerprint of the server's key. Answering "yes" will save the server's fingerprint in a local cache. Should the fingerprint ever change, ssh (and scp) will sound an alarm, as this could be an indication of a man-in-the-middle attack in progress.

* You will be prompted for your password on the Web server. Enter it, and the key file will be copied.

3) Test the ssh key:


rob@entropy$ ssh www.mydomain.net
ogin: Mon Oct 29 10:58:32 2001 from entropy.oreilly.com rob@www$

* It should log you in without a password. If not, check your work. Also check that your Web server allows public key exchange (it's on by default, and is rarely disabled. Check with your friendly local sysadmin if you're not sure.)

5) Log out, and try an scp:


rob@www$ exit
Connection to www closed.
rob@entropy$ scp /path/to/my/webcam.jpg www.mydomain.net:/path/to/graphic/
webcam.jpg           100% |*****************************|   601       00:00 
rob@entropy$

* It should copy the file without asking for a password. The syntax is: 


scp [source file] [hostname]:[destination directory]

* This example also assumes that you have the same username on your Mac OS X client as on your Web server. If not, you can also specify a different username on the scpcommand line. For example, if your login on the Web server is webadmin: 


rob@entropy$ scp /path/to/my/webcam.jpg webadmin@www.mydomain.net:/path/to/graphic/

6) Set up cron:

Before adding a new entry to cron (sometimes called your crontab), you may want to set your editor to the user-friendly editor pico by using this command: 


name% setenv EDITOR pico

Now that you've established pico as your default editor, you can set up the cron entry: 


rob@entropy$ crontab -e

(This will start pico with the contents of your current crontab. Unless you've added some lines yourself, this will probably be an empty file.)

Add this line: 


*/5 * * * * /usr/bin/scp /path/to/my/webcam.jpg www.mydomain.net:/path/

(This means, every five minutes of every day, do this...)

7) In five minutes, check your Web site with a browser. The updated picture should magically appear. Congratulations! Your webcam images are now being updated over a cryptographically secure channel, safe from wireless eavesdroppers.

When you are finished with your webcam, it's a good idea to tell cron to stop trying to update your Web site. To disable the cron entry, run another crontab -e in your terminal window, and comment out the line with a # symbol: 


# */5 * * * * /usr/bin/scp /path/to/my/webcam.jpg

Now, whenever you want to start the auto-update process again, just delete the #, and away you go.

If you have trouble getting it going, there are excellent man pages available for ssh-keygen, ssh, and scp. You might also take a look at Rob's article on using SSH with wireless networks.

Final thoughts

This probably feels like a lot of work just to set up a webcam session. But the risks of transmitting unsecured data over a public network far outweigh the effort to set up ssh/scp. The good news is, once you've established your system, it's easy to turn on and off. As a bonus, with ssh keys in place, you can securely copy files and log in to your Web server without needing to use passwords.

If you discover a clever workaround that saves time or improves performance, be sure to let us know via the TalkBacks.

Derrick Story is the author of The Photoshop CS4 Companion for Photographers, The Digital Photography Companion, and Digital Photography Hacks, and coauthor of iPhoto: The Missing Manual, with David Pogue. You can follow him on Twitter or visit www.thedigitalstory.com.

Rob Flickenger is a long time supporter of FreeNetworks and DIY networking. Rob is the author of three O'Reilly books: Building Wireless Community Networks, Linux Server Hacks, and Wireless Hacks.

Return to the Wireless DevCenter.


  • SSH Problem with Instructions
    2001-11-22 08:21:08  cochella [View]

    Great article; clear concise.

    However, I am unable to repeat the SSH public/private key login process.

    I following the instructions exactly and was always prompted for a password and not a passphrase (I chose to provide a passphrase).

    The closest I can get to being prompted for a passphrase (I also get prompted for a password after entering the passphrase) is to do a few changes:

    1. On the remote host put the public id_dsa.pub key in .ssh2/ and create a file called "authorization" that contains "Key id_dsa.pub". This is for multiple keys I believe.

    2. Edit the id_dsa.pub key on local and remote to remove "ssh-dss" at the beginning of the key file and the "username@localhost" at the end of the key file.

    After doing all of this I still get the following session:

    ssh -l chris hostname.com
    Enter passphrase for key '/Users/cochella/.ssh/identity':
    chris@hostname.com's password:
    Last login: Thu Nov 22 09:28:36 2001 from 166.70.174.201
    [chris@server1 cochella]$


    If I understand everything this means that passphrase encryption (based on the keys) did not take place and a lesser form of encryption via the password was executed. Is this correct?

    How can I fix this?

    I have been forward and backward with all possible combinations and verifying passwords.

    Thank you,

    Chris

    Local System: Mac OSX 10.1
    Remote System: RedHat 6.2 running openssh
    • SSH Problem with Instructions
      2001-11-26 19:22:35  res [View]


      
1.  On the remote host put the public id_dsa.pub key in .ssh2/ and create
      
a file called "authorization" that contains "Key id_dsa.pub".  This is for
      
multiple keys I believe.


      No; this is the convention used by the SSH server from HREF="http://www.ssh.com/">ssh.com. It will not work with OpenSSH
      (which is what you say is running on the server). And if the server
      were ssh.com, you would need to convert the key format in any
      case. 


      
2.  Edit the id_dsa.pub key on local and remote to remove "ssh-dss" at the
      
beginning of the key file and the "username@localhost" at the end of the
      
key file.


      The "username@localhost" is a comment and removing it does no harm, but
      the leading "ssh-dss" is part of the key format, and removing it will
      break the file. 


      
After doing all of this I still get the following session:
      
ssh -l chris hostname.com
      
Enter passphrase for key '/Users/cochella/.ssh/identity':
      
chris@hostname.com's password:
      
Last login: Thu Nov 22 09:28:36 2001 from 166.70.174.201
      
[chris@server1 cochella]$
      

      
If I understand everything this means that passphrase encryption (based on
      
the keys) did not take place and a lesser form of encryption via the
      
password was executed.  Is this correct?


      There are a couple bits of confusion here.



      1. The name of the key file implies that the connection is using SSH
        protocol version 1 (you can see this more specifically using ssh -v
        ...        . The key you generated can only be used with protocol 2, so
        public-key authentication will not work. Even if the server supports
        protocol 2, the version of OpenSSH which comes with OSX (2.3.0) will choose
        protocol 1 if it's available (this default was changed in OpenSSH-2.9).
        Try ssh -2 ....

      2. The client authentication method you employ has no bearing on what
        kind of encryption protects the session. All session encryption
        parameters are in fact negotiated and put into use before client
        authentication even takes place.




      
+-----------------------------------------------------------------------------+
      
| Richard Silverman | co-author: SSH, The Secure Shell (The Definitive Guide) |
      
| slade@shore.net   | O'Reilly, 2001 -- http://www.oreilly.com/catalog/sshtdg |
      
+-----------------------------------------------------------------------------+

  • SSH Problem with Instructions
    2001-11-28 13:58:11  cochella [View]

    No; this is the convention used by the SSH server from HREF="http://www.ssh.com/">ssh.com. It will not work with OpenSSH
    (which is what you say is running on the server). And if the server
    were ssh.com, you would need to convert the key format in any
    case.

    On the remote server there is OpenSSH and I was using the wrong convention; I did not realize there were different conventions--silly. The server OpenSSH version is: openssh-2.2.0p1-2

    So, I have changed conventions to thosethat your article describes: using .ssh with an authorized_keys2 file with the public key in this file on the remote machine.

    After making these changes I still only get password authentication:


    [localhost:~/.ssh] cochella% ssh -2 -l chris -i /Users/cochella/.ssh/identity cochella.com
    chris@myhostname.com's password:
    Last login: Wed Nov 28 15:01:07 2001 from 166.70.xxx.xxx
    [chris@server1 cochella]$

    As suggested I am trying to do version 2 with the switch "-2"


    So, I do not understand what the problem is?

    Any further advice would be most helpful.

    Thanks.

    Chris
    • SSH Problem with Instructions
      2001-11-28 15:20:30  res [View]


      Use "ssh -v ..." to see what it's doing.

      Why are you using -i to tell it to use the wrong key? As I pointed out last time, ~/.ssh/identity contains a protocol-1-only key.
      • SSH Problem with Instructions
        2001-12-31 07:02:22  cochella [View]

        Here is the -v output. It appears, from what I can understand, that public key authentication is taking place. But, I get asked for the password and not the passphrase.

        Thanks,

        Chris


        _________

        [localhost:~/.ssh] cochella% ssh -2 -v -l chris myhost.com
        OpenSSH_2.9p2, SSH protocols 1.5/2.0, OpenSSL 0x0090602f
        debug1: Reading configuration data /etc/ssh_config
        debug1: Seeding random number generator
        debug1: Rhosts Authentication disabled, originating port will not be trusted.
        debug1: restore_uid
        debug1: ssh_connect: getuid 501 geteuid 501 anon 1
        debug1: Connecting to myhost.com [209.55.121.12] port 22.
        debug1: restore_uid
        debug1: restore_uid
        debug1: Connection established.
        debug1: identity file /Users/cochella/.ssh/id_rsa type -1
        debug1: identity file /Users/cochella/.ssh/id_dsa type 2
        debug1: Remote protocol version 1.99, remote software version OpenSSH_2.2.0p1
        debug1: match: OpenSSH_2.2.0p1 pat ^OpenSSH[-_]2\.[012]
        Enabling compatibility mode for protocol 2.0
        debug1: Local version string SSH-2.0-OpenSSH_2.9p2
        debug1: SSH2_MSG_KEXINIT sent
        debug1: SSH2_MSG_KEXINIT received
        debug1: kex: server->client 3des-cbc hmac-md5 none
        debug1: kex: client->server 3des-cbc hmac-md5 none
        debug1: dh_gen_key: priv key bits set: 197/384
        debug1: bits set: 524/1024
        debug1: sending SSH2_MSG_KEXDH_INIT
        debug1: expecting SSH2_MSG_KEXDH_REPLY
        debug1: Host 'myhost.com' is known and matches the DSA host key.
        debug1: Found key in /Users/cochella/.ssh/known_hosts2:3
        debug1: bits set: 506/1024
        debug1: len 55 datafellows 49296
        debug1: ssh_dss_verify: signature correct
        debug1: kex_derive_keys
        debug1: newkeys: mode 1
        debug1: SSH2_MSG_NEWKEYS sent
        debug1: waiting for SSH2_MSG_NEWKEYS
        debug1: newkeys: mode 0
        debug1: SSH2_MSG_NEWKEYS received
        debug1: done: ssh_kex2.
        debug1: send SSH2_MSG_SERVICE_REQUEST
        debug1: service_accept: ssh-userauth
        debug1: got SSH2_MSG_SERVICE_ACCEPT
        debug1: authentications that can continue: publickey,password
        debug1: next auth method to try is publickey
        debug1: try privkey: /Users/cochella/.ssh/id_rsa
        debug1: try pubkey: /Users/cochella/.ssh/id_dsa
        debug1: authentications that can continue: publickey,password
        debug1: next auth method to try is password
        chris@myhost.com's password:
        debug1: ssh-userauth2 successful: method password
        debug1: channel 0: new [client-session]
        debug1: channel_new: 0
        debug1: send channel open 0
        debug1: Entering interactive session.
        debug1: client_init id 0 arg 0
        debug1: channel request 0: shell
        debug1: channel 0: open confirm rwindow 0 rmax 32768
        Last login: Mon Dec 31 08:11:09 2001 from 166.55.565.65
        [chris@server1 cochella]$
      • SSH Problem with Instructions
        2001-11-30 05:05:10  ahinds [View]

        I too am having the same problem with automatic logins.

        I followed the instructions to the letter. Here is part of my session transcript. Any help would be appreciated. Thanks!

        ---
        [localhost:~/.ssh] ahinds% ssh -v xxx.com
        OpenSSH_2.9p2, SSH protocols 1.5/2.0, OpenSSL 0x0090602f
        debug1: Reading configuration data /etc/ssh_config
        debug1: Seeding random number generator
        debug1: Rhosts Authentication disabled, originating port will not be trusted.
        debug1: restore_uid
        debug1: ssh_connect: getuid 501 geteuid 501 anon 1
        debug1: Connecting to xxx.com [xxx.xx.xxx.xx] port 22.
        debug1: restore_uid
        debug1: restore_uid
        debug1: Connection established.
        debug1: identity file /Users/ahinds/.ssh/identity type -1
        debug1: identity file /Users/ahinds/.ssh/id_rsa type -1
        debug1: identity file /Users/ahinds/.ssh/id_dsa type 2
        debug1: Remote protocol version 1.99, remote software version OpenSSH_2.9p2
        debug1: match: OpenSSH_2.9p2 pat ^OpenSSH
        Enabling compatibility mode for protocol 2.0
        debug1: Local version string SSH-2.0-OpenSSH_2.9p2
        debug1: SSH2_MSG_KEXINIT sent
        debug1: SSH2_MSG_KEXINIT received
        debug1: kex: server->client aes128-cbc hmac-md5 none
        debug1: kex: client->server aes128-cbc hmac-md5 none
        debug1: SSH2_MSG_KEX_DH_GEX_REQUEST sent
        debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
        debug1: dh_gen_key: priv key bits set: 125/256
        debug1: bits set: 1049/2049
        debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
        debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
        debug1: Host 'xxx.com' is known and matches the RSA host key.
        debug1: Found key in /Users/ahinds/.ssh/known_hosts2:1
        debug1: bits set: 1012/2049
        debug1: ssh_rsa_verify: signature correct
        debug1: kex_derive_keys
        debug1: newkeys: mode 1
        debug1: SSH2_MSG_NEWKEYS sent
        debug1: waiting for SSH2_MSG_NEWKEYS
        debug1: newkeys: mode 0
        debug1: SSH2_MSG_NEWKEYS received
        debug1: done: ssh_kex2.
        debug1: send SSH2_MSG_SERVICE_REQUEST
        debug1: service_accept: ssh-userauth
        debug1: got SSH2_MSG_SERVICE_ACCEPT
        debug1: authentications that can continue: publickey,password,keyboard-interactive
        debug1: next auth method to try is publickey
        debug1: try privkey: /Users/ahinds/.ssh/identity
        debug1: try privkey: /Users/ahinds/.ssh/id_rsa
        debug1: try pubkey: /Users/ahinds/.ssh/id_dsa
        debug1: authentications that can continue: publickey,password,keyboard-interactive
        debug1: next auth method to try is password
        ahinds@xxx.com's password:
        • SSH Problem with Instructions
          2001-12-31 06:52:52  cochella [View]

          Have you found a solution yet?

          Thanks,

          Chris
          • SSH Problem with Instructions
            2007-04-02 08:32:59  bglnelissen [View]

            i have had the same problems, it worried me the whole day, one way was doing ok, the other way was asking my password.

            But now it is fixed. My problem where the permissions of my home folder, .ssh folder and the content of the .ssh folder.
            I did a (i can be wrong, if so corrent me but i dont know the default permissions of the home folder so i took 755)

            change the rights of EVERY file in my homefolder, this is done with the -R flag. It might be nicer if you dont use it at all and type (chmod 755 /Users/USERNAME/)
            $ chmod -R 755 /Users/USERNAME/

            change the permissions of the .ssh folder
            < code >$ chmod 700 ~/.ssh

            change the permissions of the .ssh folders content
            < code >$ chmod 600 ~/.ssh/*

            good luck.